PROJECT SUMMARY Human rhinovirus (HRV) is the most common respiratory virus detected in the upper and lower respiratory tract in hematopoietic cell transplant (HCT) recipients; mortality rates following HRV lower respiratory tract infection are similar to those seen with known pulmonary viral pathogens including respiratory syncytial virus, influenza, and parainfluenza virus. Despite the high burden of disease and the observed complications of HRV infection in HCT recipients, the development of HRV therapeutics is hindered by the lack of a comprehensive understanding of the relationship between viral detection, symptoms and host immune responses and the impact of these factors on disease severity. We have demonstrated that clinical risk factors, including cytopenias and steroid use, are associated with progression from upper respiratory tract infection (URTI) to LRTI. However, up to 70% of patients with profound immunosuppression at the time of virus acquisition clear their infections without treatment. Our preliminary data show that gene expression signatures at the time of URTI may be predictive of progression to LRTI. We aim to characterize the interplay between viral detection, cytokine levels and cellular immune responses early during HRV infection in HCT recipients in a prospective surveillance cohort. We will also perform in depth gene expression analyses at a single cell level to identify specific cellular populations that are associated with severe disease in both peripheral blood and in proximal bronchoalveolar lavage fluid. The central hypothesis of this proposal is that viral and host immune kinetics, including both global and cell specific gene expression profiles in specific tissue compartments, impact disease severity in HCT recipients with HRV infection. Results from these experiments will characterize the optimal timing and most predictive viral and host immune markers that can be used to design rational clinical trials for novel therapeutics. Our deep interrogation of compartment and cell specific immune responses will further our understanding of virus-host interactions and of potential targets for intervention.